In an article in The Strategist, the author referred to Dr. Yi Cui proposing facial tissues (specifically Kleenex, but not sure if this is because of sponsorship $ or the brand is better than others) as added filters in cloth masks.
You may not think the humble tissue would do much against a pandemic virus, but Stanford materials science and engineering professor Yi Cui, who co-authored a study on the best household materials for face masks, says that while they won’t do much good on their own, tissues are actually fairly effective when used in a mask as a filter. He recommends adding two layers of tissue in the filter pocket of a cloth mask and changing the tissues out after each use. It’s definitely an affordable choice.
If tissues are a good enough boost to pass then the panty shield option looks even better as it has layers of the most effective materials mentioned in the study.
Both the Strategist article and the study itself are worth a read because they provide some good ideas for mask filters when supplies of ready made items are low. My one disagreement is with materials that are not cosmetics grade or meant for use on the skin. They may create particles that you don’t want to inhale and may cause irritation.
So for me, it’s panty shields, but if you’re not comfortable with that then at least now we know facial tissues will do an okay job.
Source: Household Materials Selection for Homemade Cloth Face Coverings and Their Filtration Efficiency Enhancement with Triboelectric Charging